DOHA, Qatar (CNN) --Coalition forces searched Tuesday for two crew members of a U.S. Air Force F-15E Strike Eagle fighter that went down over Iraq, U.S. military officials said.

The plane went down Monday in the vicinity of Tikrit, Saddam Hussein's ancestral homeland and an area where the U.S.-led coalition does not have many forces.

The plane went down at approximately 3:30 a.m. local time Monday (7:30 p.m. EDT Sunday), the U.S. Central Command said.

Missing were the plane's pilot and its weapons officer, officials said.

"The cause of the incident is unknown at this time and is being investigated," Central Command said.

The crew members and their plane were from the 4th Fighter Wing at Seymour Johnson Air Force Base, North Carolina, the statement said.

The statement added that coalition forces do not discuss search and rescue operations, and reminded "Iraqi forces and Iraqi citizens they must abide by all Geneva Conventions relating to the treatment of prisoners. Violations of these laws will bring swift and severe consequences."